The invention relates to a floating wind energy generating device comprising at least two wind turbines mounted on a floating open structure, each of said at least two wind turbines featuring a blade and rotor part mounted at the top of a tower, said floating open structure being moored to the seabed with a mooring system and connected to a single high voltage power export cable for exporting the electricity generated by the at least two windmills.
Device of this kind are known from literature, wherein various proposals have been made as regards to the construction of such a device. It is not known, in particular for wind turbines having large dimensions, how the various problems related to its installation are solved.
A typical state-of-the-art solution for securing offshore wind turbines was to place them on special foundations, which are either lowered onto the sea floor or forced into the seabed. Further, current offshore wind industry practice is to erect large wind turbines designed for onshore use, under complex maritime conditions offshore with the aid of a dedicated jack-up barge aimed at creating a stable working platform for the operations offshore. The currently used dedicated installation equipment poses a risk of major delays in installation works, for example when the jack-up barge encounters a breakdown.
The non-floating offshore wind turbine concepts point to a series of disadvantages such as commissioning, repair and maintenance of the wind turbines which needs to be carried out offshore. This includes the high costs associated with offshore work due to travel, distance, downtime and removal of such foundations after closure and decommissioning of the wind farm. These can be vulnerable to bad weather conditions and by poor installation accessibility.
Another key issue is the substantial risk of extended installation downtime during high winds. Under these conditions dynamic turbine loads are at maximum, yields most favorable, and simultaneously the implications of a breakdown are most severe due to difficult turbine access.
Decommissioning and compulsory removal of permanent foundations at the end of a wind farm's operational service life is also widely considered to be an underestimated cost associated with fixed offshore wind turbines, for which substantial sums must be set aside.
Floating offshore wind turbines can be installed at sites with much greater water depths, out of sight from onshore, with less interference with bird migration, and increased power production due to stronger and more stable wind conditions. In addition, the floating concept might enable the use of conventional, non-dedicated tugboats, which are readily available in any market. This limits possible downtime to a minimum. It is therefore an object of the present invention to propose a floating wind turbines system.
Prior art patent application WO2006/121337 discloses a floating wind turbine installation for one floating wind turbine, comprising a long, submerged floating concrete cylinder that is ballasted. The individual anchor lines are each, at a certain distance from the floating cylinder at a fixing point on the individual anchor line, connected with double lines connected to the floating cylinder in a delta-shaped arrangement. This configuration is hence limited to single wind turbines for deep waters as this prominent feature, the ballasted concrete cylinder, makes the design unsuitable for shallower waters. The present invention proposes a system that is suitable for multiple wind turbines for deep and shallower waters.
Patent application US2008089746 discloses a floating platform for a single wind turbine that can be installed in shallow or deep waters. The platform is fixed in position using strong chains linked to heavy weights on the seabed. By changing the length of the chains, it can be installed at very different depths. The platform comprises a steel structure with six interconnected hollow steel pipes providing the necessary buoyancy required during sea transport, as well as during offshore operation. The wind turbine itself is located on a tubular steel tower on the platform. Once the platform with the fully assembled turbine has arrived at its destination, the assembly is temporarily ballasted. This extra ballast forces the platform down into the water and creates the free play necessary to hook the six chains attached to the counterweight to the platform. When the extra ballast is removed, buoyancy creates upward force whereby the chains get tensioned in order to provide some stability. In this configuration, the necessary buoyancy required for sea transportation is integrated in the platform for one single wind turbine.                Patent NL1008318 discloses a wind turbine island comprising several wind turbines equidistant one to the other and placed on a hub anchored to the sea bed by cables. Each wind turbine rotor is on the top of a pillar which are linked by horizontal concrete tubes and fastened to the hub by radial fixing cables. A drive mechanism rotates to bring the rotors into the wind. Nothing is mentioned about the installation method, which is a key point in designing a floating system that needs to be transported over the sea.        Patent WO2007/009464 which discloses a plant for exploiting wind energy at sea comprising several wind turbines which are installed on a structure comprising longitudinal beams, support means and anchoring means. The structure is also supplied with buoyancy elements provided with ballast, and the plant is adapted so that it can adjust depending on the direction of the wind. In this invention, the plant is a floatable structure, carried by the buoyancy of buoyancy elements or pontoons in the water. During towing, the complete buoyancy of the pontoons is used, hence during towing the plant is lying high in the water, with all beams above sea water level as well as most of the support structure. In this configuration the stability is very weak, the center of gravity is well above of the center of buoyancy and the structure is permanently menacing of tilting. The stability is obtained only once the plant is brought out to the production site, when buoyancy means are ballasted until there is obtained such a balance between buoyancy and mass.        
Norwegian engineering consultancy company Force Technology has developed an offshore wind unmanned floating structure which is self-orientating towards the wind and, accommodates three wind turbines mounted at each corner of a floating triangular lattice-type welded steel foundation structure. The foundation structure is anchored to the seabed and can rotate as the wind changes direction. However, in such a configuration, wind turbines are too near one from the other and even if one is higher than the other, their efficiency will be limited and not optimal.
U.S. Pat. No. 6,294,844 discloses a weathervaning wind energy converter comprising wind turbines which are mounted in a frame provided with floating bodies and supporting means which are disposed some distance away from the plane of the wind turbines to keep it substantially vertically oriented. However, this overall costly and high construction with long blades, has an very high center of gravity which results in unstable behavior of the floating system in rough weather conditions.
Patent application WO02/073032 discloses an offshore floating wind power generation plant with single point mooring system fixing a floater to the sea floor, one or several wind power generation unit being placed on the floater. The float is always directed at a constant orientation to the wind, the plant being horizontally rotatable about the mooring point. In this configuration, the fatigue and stress applied on the electrical cable are important, as even if the structure is quite stable, the movement of the structure due to the wave and wind will have a large fatigue effect on the cable, especially at the connection point.
To increase the production and make the investment worthwhile, many existing designs of offshore wind turbines are using a single very large wind turbine generator. However, the installation, maintenance and repair of very large offshore wind turbines are very costly operations and are often depending on special designed, dedicated installation vessels.
It is a therefore an objective of the present invention to provide a floating system that ensure sufficient overall stability during sea transport, which can withstand the huge dynamic loads and acceleration forces imposed upon the structure in a floating transport mode when it is installed. It is another objective of the present invention to provide a cost effective solution and method with regard to maintenance and installation costs of a floating wind turbine system. It is another objective of the present invention to provide a system and installation method that combines well known and proven technologies from the offshore industry. It is also an objective of the present invention to provide a design for a floating offshore wind turbine system that avoids fatigue problems of the electricity cables.
The present invention provides a floating energy generating device that comprises at least two wind turbines mounted on a floating open structure, said floating open structure being moored to the seabed with a mooring system, each wind turbine featuring in operation mode a blade and rotor part mounted at the top of a tower and being connected via a cable to a central control power electronic unit on the floating open structure, the central control power unit being connected to a submerged high voltage power export cable for exporting the electricity generated by the at least two windmills, the floating open structure being connectable to or disconnectable from the mooring system and when disconnected from the mooring system the center of gravity of the floating open structure being at or below its center of buoyancy.
The present invention also provides an installation method for an offshore floating wind energy generating device according to the present invention.
The present invention further provides a cost effective solution for a wind energy converter system that ensures sufficient overall stability during sea transport. The use of multi smaller-scale offshore wind turbines is also a way to increase the production. Even if having more wind turbines can imply to have more often maintenance and reparation, these operations become far less costly. In fact, the production does not have to be stopped for the change-out of one wind turbine. Also the transport at sea is much easier as compared with a floater for a very big turbine, as several smaller ones provide for a center of gravity of the floater which is much lower than the center of gravity of one very large floating windmill. Hence the stability during transport and during operation is enhanced. The stability during transport can be enhanced by lowering even more the center of gravity, by ballasting the support structure, by the use of vertical axis windmills, or in the case of horizontal axis wind turbines, to transport them without the rotor and blades.